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A copula is a linking verb: it does not show an action, it joins the subject to what you are saying about it. In 'The house seems big', seems is the copula — it links house to big. Be is the most common one (is, are, was, were). Seem, become, appear, feel, look, and remain can work this way too, though actually grammar teachers do not all draw the line in the same place, and some lists are tidier than real sentences. You meet this word in grammar class, not in the street. It is a teacher's and linguist's word. The everyday name in Taiwan classrooms is often 連綴動詞 — same idea, friendlier clothes.
連綴動詞；系詞（語言學用語）
原始詞典資料5 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
copulanoun/ˈkɒpjʊlə/
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grammarhuman-scienceslinguisticsA word, usually a verb, used to link the subject of a sentence with a predicate (usually a subject complement or an adverbial), that unites or associates the subject with the predicate.
- I begin by arguing in section 2 that there are in fact at least two Celtic copulas, a grammatical copula that simply spells out tense and agreement, and a substantive copula formed on a lexically listed verbal stem.
- The theory of conjunctively tensed copulae will be developed and stated with more precision in the following section.
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The bond or relationship by which two things are combined into a unity.
- The fact that in milk the copula or bond is exceedingly slight is evident from the spontaneous resolution of milk when left in a vessel, its resolution, namely, into cream—a white substance of comparative consistency,—and a sourish fluid; also from its ready resolution when, merely by motion, it is turned into butter, or, by heat, into various kinds of curds.
- I quite correctly defined logical copulation by means [of] the copula of inclusion.
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mathematicssciencesstatisticsA function that represents the association between two or more variables, independent of the individual marginal distributions of the variables.
- In 2000, David X. Li, a banker with a doctorate in statistics who was then at RiskMetrics, part of J. P. Morgan Chase, began using mathematical functions called Gaussian copulas to estimate the likelihood of corporations’ dying in unison.
- There is little statistical theoretical theory for copulas. Sensitivity studies of estimation procedures and goodness-of-fit tests for copulas are unknown.
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entertainmentlifestylemusicA device that connects two or more keyboards of an organ.
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biologynatural-sciencesThe act of copulation; mating.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。